1. Field of the Invention
The present invention relates to radar and, more particularly, is concerned with radar transceivers employing circularly polarized waveforms.
Most objects reflect radio waves. Radar systems detect reflected radio waves to "see" objects such as aircraft, ships, vehicles, structures, and automobiles during the day or night through rain, fog or clouds. A classic radar system consists of a radio transmitter, radio receiver, two antennas, and a display. A radar system having both a transmitter and a receiver is called a radio transceiver. The radio transmitter generates continuous or pulsed radio waves by a voltage controlled oscillator. The radio waves are in turn radiated by one of the two antennas. Portions of the transmitted waves that are reflected back from objects are picked up by the other antenna and translated in the receiver by a mixer from their microwave frequency to a lower intermediate frequency for further signal processing. Because this radar transceiver employs two antennas, however, it is large and heavy. Furthermore, receiver sensitivity is seriously degraded if the reflected signal has the same polarization as the transmitted signal. This sensitivity reduction must be compensated with an increased transmitter power.
Another conventional radar system employes a circulator which permits the use of a single antenna for both receiving and transmitting. A voltage controlled oscillator generates a continuous wave signal which is directed to the antenna through the circulator. The single antenna radiates a microwave signal and receives the return echos. The return signals are directed through the circulator to a mixer where the signal is reduced in frequency for further signal processing. The magnetic materials such as ferrite used in a circulator, however, are not compatible with monolithic integrated circuits. The circulator is also susceptible to shock and vibration.
A further single antenna radar system developed can eliminate the circulator. A signal generated by a voltage controlled oscillator is fed into a 3-dB branch line coupler. A linear antenna is connected to the second branch of the coupler while the third branch of the coupler is terminated. Therefore, half of the generated signal passes through the coupler to the antenna while the other half of the signal is dumped at the third branch. The linear antenna radiates the signal from the second branch and receives any return signal. The return signal enters the second branch of the coupler and is split in two, one-half of which is directed toward a mixer, the other half of which is directed toward the signal generator and thus not used. Although this configuration has the advantage of using one antenna, about three-quarters of the power is wasted in operation.